1. Field of the Invention
The present invention relates to a laser beam welding method for carbon steels of different carbon content.
2. Description of Related Art
Heretofore, as a welding technology for vehicle wheels, the laser beam welding method as disclosed in patent application No. H8-28358 is proposed by the applicant. In this method, in order to reduce time in manufacturing and to obtain processing efficiency, when welding a vehicle wheel of four component parts comprising a rim, a spoke, a hub and a boss, joint regions between the rim and the spoke, between the spoke and the hub, and between the hub and the boss, for example, are welded to each other by a laser beam welding method which is less affected by thermal strain or the like.
By the way, the component parts such as the rim, the spoke and the hub or the like are formed of rolled steel plate of less carbon content for obtaining good workability or the like, while the boss which bears a direct load from the wheel axle is formed of carbon steel for machine structural use which contains more carbon and is harder than the rolled steel plate. In the case of laser beam welding the hub with the boss of different carbon content therefrom, when applying the laser beam perpendicularly to the joint region, hardening areas are apt to be produced with respect to the welded region. Namely, as seen in FIGS. 4(A), 4(B), when the laser beam is applied perpendicularly to the joint region between a cylindrical boss 51 and a hub 52 from the direction of the arrow in FIG. 4(A) so as to join them to each other by fusing and welding the end of the boss 51 into the welded region, there may be cases like that shown in FIG. 4(B) where a hardening area c is produced in the welded region y. This hardening area c will be caused due to reasons such as when the laser beam is applied to the side of the steel having a greater carbon content, a portion having a greater carbon content and a portion having a lower carbon content are generated with respect to fused metal in the welded region through the application of the laser beam welding comprising rapid heating and rapid quenching and such as when the welded metal is solidified, the portion of greater carbon content stays there as the hardening area.